Scientists and others are still trying to figure that out. It's not something one just chooses to do. It must have something to do with peoples' brain chemistry - how they react to different people. Scientists are trying to figure out how much of it is in our genes.

I don't think that at this point science has a definitive answer to this question, but here are two factors that, among others, are relevant: Everybody, both male and female, have in their body, both male and female hormones.These hormones are responsible for regulating the sexual activity of adults, and are very important in the changes that our bodies experience through puberty, both for boys and for girls. An unusually high content of hormones of the opposite sex in the body of a person can make that person tend to "think", feel and behave as a person of the opposite sex, therefore wanting to have sexual relationships with people of their own gender. Whenever two people are together in a love relationship, there are more than physical needs that need to be satisfied; for different people, these needs may be different, but for most humans, being understood, being respected, sharing common interests, and just plain having fun together are equally important than the physical aspect of the relationship. Some people, due to their education, past experiences in life, or just simple because that's the way their minds work, feel that these needs can only be met by somebody of their same gender.

These are just two reasons, one biological, one psychological, and they are probably quite oversimplified (I apologized for that), but may shed some light on the question, at the same time that they show that it's not easy to answer it in a simple, definitive way.

Different people have different ideas about this, but in science, no one really knows why.

Well, just this question being asked by scientists is controversial to some people. Some people feel that this is valid question for science to try to answer and others do not. At this time, science does not really have an answer to this question, but maybe it's not a question that science can answer. People are what they are and there is nothing wrong, abnormal or really different about people who are gay and lesbian. Everyone is just human like everyone else.